ALSTOM T&D. Network Protection And Automation Guide (NPAG)

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Chapter 13  Protection of Complex Transmission Circuits

13-17

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Alstom Grid 14-1

Chapter 14

Auto-Reclosing

14.1 Introduction

14.2 Application of Auto-Reclosing

14.3 Auto-Reclosing on HV Distribution Networks

14.4 Factors Influencing HV Auto-Reclose Schemes

14.5 Auto-Reclosing on EHV Transmission Lines

14.6 High Speed Auto-Reclosing on EHV Systems

14.7 Single-Phase Auto-Reclosing

14.8 High-Speed Auto-Reclosing on Lines Employing

Distance Schemes

14.9 Delayed Auto-Reclosing on EHV Systems

14.10 Operating Features of Auto-Reclose Schemes

14.11

Auto-Close Schemes

14.12 Examples of Auto-Reclose Applications

14.1 INTRODUCTION

Faults on overhead lines fall into one of three categories:

x transient

x semi-permanent

x permanent

80-90% of faults on any overhead lin

e network are transient in

nature. The remaining 10%-20% of faults are either semi-

permanent or permanent.

Transient faults are commonly caused by lightning or

temporary contact with foreign objects, and immediate

tripping of one or more circuit breakers clears the fault.

Subsequent re-energisation of the line is usually successful.

A small tree branch falling on the line could cause a semi-

permanent fault. The cause of the fault would not be removed

by the immediate tripping of the circuit, but could be burnt

away during a time-delayed trip. HV overhead lines in forest

areas are prone to this type of fault. Permanent faults, such as

broken conductors, and faults on underground cable sections,

must be located and repaired before the supply can be

restored.

Use of an auto-reclose scheme to re-energise the line after a

fault trip permits successful re-energisation of the line.

Sufficient time must be allowed after tripping for the fault arc

to de-energise before reclosing otherwise the arc will re-strike.

Such schemes have been the cause of a substantial

improvement in continuity of supply. A further benefit,

particularly to EHV systems, is the maintenance of system

stability and synchronism.

A typical single-shot auto-reclose scheme is shown in Figure

14.1 and Figure 14.2. Figure 14.1 shows a successful

reclosure in the event of a transient fault, and Figure 14.2 an

unsuccessful reclosure followed by lockout of the circuit

breaker if the fault is permanent.

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Network Protection & Automation Guide

14-2

Figure 1

: Singl

shot aut

reclose scheme operation for a permanent faul

Figure 1

: Singl

shot aut

reclose scheme operation for a transient faul

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Chapter 14  Auto-Reclosing

14-3

14.2 APPLICATION OF AUTO-RECLOSING

The most important parameters of an auto-reclose scheme

are:

x dead time

x reclaim time

x single or multi-shot

These parameters are influen

ced by:

x type of protection

x type of switchgear

x possible stability problems

x effects on the various types of consumer loads

The weighting given to the above factors is differen

t for HV

distribution networks and EHV transmission systems and

therefore it is convenient to discuss them under separate

headings. Sections 14.3 and 14.4 cover the application of

auto-reclosing to HV distribution networks while Sections 14.5

to 14.9 cover EHV schemes.

The rapid expansion in the use of auto-reclosing has led to the

existence of a variety of different control schemes. The various

features in common use are discussed in Section 14.10. The

related subject of auto-closing, that is, the automatic closing of

normally open circuit breakers, is dealt with in Section 14.11.

14.3 AUTO-RECLOSING ON HV DISTRIBUTION

NETWORKS

On HV distribution networks, auto-reclosing is applied mainly

to radial feeders where problems of system stability do not

arise, and the main advantages to be derived from its use can

be summarised as follows:

x reduction to a minimum of the interruptions of supply

to the co

nsumer

x instantaneous fault clearance can be introduced, with

the accompanying benefi

ts of shorter fault duration,

less fault damage, and fewer permanent faults

As 80% of overhead line faults are transient, elimination of loss

of supply from this cause by the introduction of auto-reclosing

gives obvious benefits through:

x improved supply continuity

x reduction of substation visits

Instantaneous trippi

ng reduces the duration of the power arc

resulting from an overhead line fault to a minimum. The

chance of permanent damage occurring to the line is reduced.

The application of instantaneous protection may result in non-

selective tripping of a number of circuit breakers and an

ensuing loss of supply to a number of healthy sections. Auto-

reclosing allows these circuit breakers to be reclosed within a

few seconds. With transient faults, the overall effect would be

loss of supply for a very short time but affecting a larger

number of consumers. If only time-graded protection without

auto-reclose were used, a smaller number of consumers might

be affected, but for a longer time period.

When instantaneous protection is used with auto-reclosing,

the scheme is normally arranged to inhibit the instantaneous

protection after the first trip. For a permanent fault, the time-

graded protection will give discriminative tripping after

reclosure, resulting in the isolation of the faulted section.

Some schemes allow a number of reclosures and time-graded

trips after the first instantaneous trip, which may result in the

burning out and clearance of semi-permanent faults. A further

benefit of instantaneous tripping is a reduction in circuit

breaker maintenance by reducing pre-arc heating when

clearing transient faults.

When considering feeders that are partly overhead line and

partly underground cable, any decision to install auto-reclosing

would be influenced by any data known on the frequency of

transient faults. Where a significant proportion of faults are

permanent, the advantages of auto-reclosing are small,

particularly since reclosing on to a faulty cable is likely to

aggravate the damage.

14.4 FACTORS INFLUENCING HV AUTO-

RECLOSE SCHEMES

The factors that influence the choice of dead time, reclaim

time, and the number of shots are now discussed.

14.4.1 Dead Time

Several factors affect the selection of system dead time as

follows:

x system stability and synchronism

x type of load

x CB characteristics

x fault path de-ionisation time

x protection reset time

These factors are discussed in the following sectio

ns.

14.4.1.1 System stability and synchronism

To reclose without loss of synchronism after a fault on the

interconnecting feeder, the dead time must be kept to the

minimum permissible consistent with de-ionisation of the fault

arc. Other time delays that contribute to the total system

disturbance time must also be kept as short as possible. The

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Network Protection & Automation Guide

14-4

problem arises only on distribution networks with more than

one power source, where power can be fed into both ends of

an inter-connecting line. A typical example is embedded

generation (see Chapter 17), or where a small centre of

population with a local diesel generating plant may be

connected to the rest of the supply system by a single tie-line.

The use of high-speed protection, such as unit protection or

distance schemes, with operating times of less than 0.05s is

essential. The circuit breakers must have very short operation

times and then be able to reclose the circuit after a dead time

of the order of 0.3s - 0.6s to allow for fault-arc de-ionisation.

It may be desirable in some cases to use synchronism check

logic, so that auto-reclosing is prevented if the phase angle has

moved outside specified limits. The matter is dealt with more

fully in Section 14.9.

14.4.1.2 Type of load

On HV systems, the main problem to be considered in relation

to dead time is the effect on various types of consumer load.

x Industrial consumers: Most industrial consumers

operate mixed loads co

mprising induction motors,

lighting, process control and static loads. Synchronous

motors may also be used. The dead time has to be long

enough to allow motor circuits to trip out on loss of

supply. Once the supply is restored, restarting of drives

can then occur under direction of the process control

system in a safe and programmed manner, and can

often be fast enough to ensure no significant loss of

production or product quality

x Domestic consumers: It is improbable that expensive

processes or dangerous conditions will be involved with

mestic consumers and the main consideration is that

of inconvenience and compensation for supply

interruption. A dead time of seconds or a few minutes

is of little importance compared with the loss of cooking

facilities, central heating, light and audio/visual

entertainment resulting from a longer supply failure

that could occur without auto-reclosing

14.4.1.3

Circuit breaker characteristics

The time delays imposed by the circuit breaker during a

tripping and reclosing operation must be taken into

consideration, especially when assessing the possibility of

applying high speed auto-reclosing.

x Mechanism resetting time: Most circuit breakers are

‘trip free’, which me

ans that the breaker can be tripped

during the closing stroke. After tripping, a time of the

order of 0.2s must be allowed for the trip-free

mechanism to reset before applying a closing impulse.

Where high speed reclosing is required, a latch check

interlock is desirable in the reclosing circuit

x Closing time: This is the time interval between the

energisation of the c

losing mechanism and the making

of the contacts. Owing to the time constant of the

solenoid and the inertia of the plunger, a solenoid

closing mechanism may take 0.3s to close. A spring-

operated breaker, on the other hand, can close in less

than 0.2s. Modern vacuum circuit breakers may have a

closing time of less than 0.1s

The circuit breaker mechanism imposes a minimum dead time

made up from the sum of (a) and (b) above. Figure

14.3

shows the performance of modern HV/EHV circuit breakers in

this respect. Older circuit breakers may require longer times

than those shown.

Trip

initiation

Time (s)

Contacts

separate

extinguished

Arc

closing circuit energised

Breaker fully open:

Breaker

fully

closed

Contacts

make

Oil

11kV

0.06 0.038 0.03 0.035 0.04 0.02

0.1 0.053 0.06 0.045 0.07 0.05

0.08 0.023 0.2 0.235 0.03 0.01

0.16 0.048 0.35 0.065 0.08 0.06

0.24 0.28 0.55 0.3 0.11 0.07

0.02 0.07 0.01 0.02 0.12 0.04

Note: 380kV data applicable to 400kV also. All times in seconds

Vacuum

15kV

Oil

132kV

Air

380kV

132kV 380kV

Figure 14.3: Typical circuit breaker trip-close operation times

14.4.1.4

De-ionisation of the fault path

As mentioned above, successful high speed reclosure requires

the interruption of the fault by the circuit breaker to be

followed by a time delay long enough to allow the ionised air to

disperse. This time is dependent on the system voltage, cause

of fault, weather conditions and so on, but at voltages up to

66kV, 0.1s - 0.2s should be adequate. On HV systems,

therefore, fault de-ionisation time is of less importance than

circuit breaker time delays.

14.4.1.5 Protection reset time

If time delayed protection is used, it is essential that the timing

device shall fully reset during the dead time, so that correct

time discrimination will be maintained after reclosure on to a

fault. The reset time of the electromechanical I.D.M.T. relay is

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Chapter 14  Auto-Reclosing

14-5

10 seconds or more when on maximum time setting, and dead

times of at least this value may be required.

When short dead times are required, the protection relays

must reset almost instantaneously, a requirement that is easily

met by the use of static, digital and numerical I.D.M.T. relays.

14.4.2 Reclaim Time

Factors affecting the setting of the reclaim time are discussed

in the following sections.

14.4.2.1 Type of protection

The reclaim time must be long enough to allow the protection

relays to operate when the circuit breaker is reclosed on to a

permanent fault. The most common forms of protection

applied to HV lines are I.D.M.T. or definite time over-current

and earth-fault relays. The maximum operating time for the

former with very low fault levels could be up to 30 seconds,

while for fault levels of several times rating the operating time

may be 10 seconds or less.

In the case of definite time protection, settings of 3 seconds or

less are common, with 10 seconds as an absolute maximum.

It has been common practice to use reclaim times of 30

seconds on HV auto-reclose schemes. However, there is a

danger with a setting of this length that during a

thunderstorm, when the incidence of transient faults is high,

the breaker may reclose successfully after one fault, and then

trip and lock out for a second fault within this time. Use of a

shorter reclaim time of, say, 15 seconds may enable the

second fault to be treated as a separate incident, with a further

successful reclosure.

Where fault levels are low, it may be difficult to select I.D.M.T.

time settings to give satisfactory grading with an operating

time limit of 15 seconds, and the matter becomes a question

of selecting a reclaim time compatible with I.D.M.T.

requirements.

It is common to fit sensitive earth-fault protection to

supplement the normal protection to detect high resistance

earth faults. This protection is usually set to have an operating

time longer than that of the main protection. This longer time

may have to be taken into consideration when deciding on a

reclaim time. A broken overhead conductor in contact with dry

ground or a wood fence may cause this type of fault. It is

rarely if ever transient and may be a danger to the public. It is

therefore common practice to use a contact on the sensitive

earth fault relay to block auto-reclosing and lock out the circuit

breaker.

Where high-speed protection is used, reclaim times of 1

second or less would be adequate. However, such short times

are rarely used in practice, to relieve the duty on the circuit

breaker.

14.4.2.2 Spring winding time

The reclaim time of motor-wound spring-closed breakers must

be at least as long as the spring winding time, to ensure that

the breaker is not subjected to a further reclosing operating

with a partly wound spring.

14.4.3 Number of Shots

There are no definite rules for defining the number of shots for

any particular auto-reclose application, but a number of factors

must be taken into account.

14.4.3.1 Circuit breaker limitations

Important considerations are the ability of the circuit breaker to

perform several trip and close operations in quick succession

and the effect of these operations on the maintenance period.

Maintenance periods vary according to the type of circuit

breaker used and the fault current broken when clearing each

fault. Use of modern numerical relays can assist, as they often

have a CB condition-monitoring feature included that can be

arranged to indicate to a Control Centre when maintenance is

required. Auto-reclose may then be locked out until

maintenance has been carried out.

14.4.3.2 System conditions

If statistical information on a particular system shows a

moderate percentage of semi-permanent faults that could be

burned out during 2 or 3 time-delayed trips, a multi-shot

scheme may be justified. This is often the case in forest areas.

Multi-shot schemes may also be applicable where fused ‘tees’

are used and the fault level is low, since the fusing time may

not discriminate with the main I.D.M.T. relay. The use of

several shots will heat the fuse to such an extent that it would

eventually blow before the main protection operated.

14.5 AUTO-RECLOSING ON EHV

TRANSMISSION LINES

The most important consideration in the application of auto-

reclosing to EHV transmission lines is the maintenance of

system stability and synchronism. The problems involved are

dependent on whether the transmission system is weak or

strong. With a weak system, loss of a transmission link may

lead quickly to an excessive phase angle across the CB used for

re-closure, thus preventing a successful re-closure. In a

relatively strong system, the rate of change of phase angle will

be slow, so that delayed auto-reclose can be successfully

applied.

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Network Protection & Automation Guide

14-6

An illustration is the interconnector between two power

systems as shown in Figure 14.4. Under healthy conditions,

the amount of synchronising power transmitted,

, crosses the

power/angle curve

OAB

at point

, showing that the phase

displacement between the two systems is



. Under fault

conditions, the curve

OCB

is applicable, and the operating

point changes to

. Assuming constant power input to both

ends of the line, there is now an accelerating power

. As a

result, the operating point moves to

, with an increased phase

displacement,



, between the two systems. At this point the

circuit breakers trip and break the connection. The phase

displacement continues to increase at a rate dependent on the

inertia of the two power sources. To maintain synchronism,

the circuit breaker must be reclosed in a time short enough to

prevent the phase angle exceeding



. This angle is such that

the area (2) stays greater than the area (1), which is the

condition for maintenance of synchronism.

Figure 14.4: Effect of high-speed three-phase auto-reclosing on

system stability for a weak system

This example, for a weak system, shows that the successful

application of auto-reclosing in such a case needs high-speed

protection and circuit breakers, and a short dead time. On

strong systems, synchronism is unlikely to be lost by the

tripping out of a single line. For such systems, an alternative

policy of delayed auto-reclosing may be adopted. This enables

the power swings on the system resulting from the fault to

decay before reclosure is attempted.

The various factors to be considered when using EHV auto-

reclose schemes are now dealt with. High-speed and delayed

auto-reclose schemes are discussed separately.

14.6 HIGH SPEED AUTO-RECLOSING ON EHV

SYSTEMS

The first requirement for the application of high-speed auto-

reclosing is knowledge of the system disturbance time that can

be tolerated without loss of system stability. This will normally

require transient stability studies to be conducted for a defined

set of power system configurations and fault conditions. With

knowledge of protection and circuit breaker operating

characteristics and fault arc de-ionisation times, the feasibility

of high-speed auto-reclosing can then be assessed. These

factors are now discussed.

14.6.1 Protection Characteristics

The use of high-speed protection equipment, such as distance

or unit protection schemes, giving operating times of less than

40ms, is essential. In conjunction with fast operating circuit

breakers, high-speed protection reduces the duration of the

fault arc and thus the total system disturbance time.

It is important that the circuit breakers at both ends of a fault

line should be tripped as rapidly as possible. The time that the

line is still being fed from one end represents an effective

reduction in the dead time, and may well jeopardise the

chances of a successful reclosure. When distance protection is

used, and the fault occurs near one end of the line, special

measures have to be adopted to ensure simultaneous tripping

at each end. These are described in Section 14.8.

14.6.2 De-Ionisation of the Fault Arc

It is important to know the time that must be allowed for

complete de-ionisation of the arc, to prevent the arc restriking

when the voltage is re-applied.

The de-ionisation time of an uncontrolled arc, in free air

depends on the circuit voltage, conductor spacing, fault

currents, fault duration, wind speed and capacitive coupling

from adjacent conductors. Of these, the circuit voltage is the

most important, and as a general rule, the higher the voltage

the longer the time required for de-ionisation. Typical values

for three-phase faults are given in Table 14.1.

Line Voltage (kV) Minimum De-energisation Time (Seconds)

66 0.2

110 0.28

132 0.3

220 0.35

275 0.38

400 0.45

525 0.55

Table 14.1: Fault-arc de-ionisation times

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Chapter 14  Auto-Reclosing

14-7

If single-phase tripping and auto-reclosing is used, capacitive

coupling between the healthy phases and the faulty phase

tends to maintain the arc and hence extend the dead time

required from the values given in the Table. This is a particular

problem on long distance EHV transmission lines. However

where shunt compensation is applied, a neutral reactor can

often be used to balance system inter-phase capacitance and

thus reduce the arcing time.

14.6.3 Circuit Breaker Characteristics

The high fault levels involved in EHV systems impose a very

severe duty on the circuit breakers used in high-speed auto-

reclose schemes. The accepted breaker cycle of break-make-

break requires the circuit breaker to interrupt the fault current,

reclose the circuit after a time delay of upwards of 0.2s and

then break the fault current again if the fault persists. The

types of circuit breaker commonly used on EHV systems are

oil, air blast and SF

types.

14.6.3.1 Oil circuit breakers

Oil circuit breakers are used for transmission voltages up to

300kV, and can be subdivided into the two types: ‘bulk oil’ and

‘small oil volume’. The latter is a design aimed at reducing the

fire hazard associated with the large volume of oil contained in

the bulk oil breaker.

The operating mechanisms of oil circuit breakers are of two

types, ‘fixed trip’ and ‘trip free’, of which the latter is the most

common. With trip-free types, the reclosing cycle must allow

time for the mechanism to reset after tripping before applying

the closing impulse.

Special means have to be adopted to obtain the short dead

times required for high-speed auto-reclosing. Various types of

tripping mechanism have been developed to meet this

requirement.

The three types of closing mechanism fitted to oil circuit

breakers are:

x solenoid

x spring

x pneumatic

CBs with solenoid closing are not suita

ble for high-speed auto-

reclose due to the long time constant involved. Spring,

hydraulic or pneumatic closing mechanisms are universal at

the upper end of the EHV range and give the fastest closing

time. Figure 14.3 shows the operation times for various types

of EHV circuit breakers, including the dead time that can be

attained

14.6.3.2 Air blast circuit breakers

Air blast breakers have been developed for voltages up to the

highest at present in use on transmission lines. They fall into

two categories:

x pressurised head circuit breakers

x non-pressurised head circuit breakers

In pressurised head circ

uit breakers, compressed air is

maintained in the chamber surrounding the main contacts.

When a tripping signal is received, an auxiliary air system

separates the main contacts and allows compressed air to

blast through the gap to the atmosphere, extinguishing the

arc. With the contacts fully open, compressed air is

maintained in the chamber.

Loss of air pressure could result in the contacts reclosing, or, if

a mechanical latch is employed, restriking of the arc in the de-

pressurised chamber. For this reason, sequential series

isolators, which isolate the main contacts after tripping, are

commonly used with air blast breakers. Since these are

comparatively slow in opening, their operation must be

inhibited when auto-reclosing is required. A contact on the

auto-reclose relay is made available for this purpose.

Non-pressurised head circuit breakers are slower in operation

than the pressurised head type and are not usually applied in

high-speed reclosing schemes.

14.6.3.3 SF

circuit breakers

Most EHV circuit breaker designs now manufactured use SF

gas as an insulating and arc-quenching medium. The basic

design of such circuit breakers is in many ways similar to that

of pressurised head air blast circuit breakers. Voltage

withstand capability depends on a minimum gas pressure

being available, and gas pressure monitors are fitted and

arranged to block CB operation in the event of low gas

pressure occurring. Sequential series isolators are normally

used, to prevent damage to the circuit breaker in the event of

voltage transients due to lightning strikes, etc. occurring when

the CB is open. Provision should therefore be made to inhibit

sequential series isolation during an auto-reclose cycle.

14.6.4 Choice of Dead Time

At voltages of 220kV and above, the de-ionisation time will

probably dictate the minimum dead time, rather than any

circuit breaker limitations. This can be deduced from Table

14.1. The dead time setting on a high-speed auto-reclose

relay should be long enough to ensure complete de-ionisation

of the arc. On EHV systems, an unsuccessful reclosure is more

detrimental to the system than no reclosure at all.

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Network Protection & Automation Guide

14-8

14.6.5 Choice of Reclaim Time

Where EHV oil circuit breakers are concerned, the reclaim time

should take account of the time needed for the closing

mechanism to reset ready for the next reclosing operation.

14.6.6 Number of Shots

High-speed auto-reclosing on EHV systems is invariably single

shot. Repeated reclosure attempts with high fault levels would

have serious effects on system stability, so the circuit breakers

are locked out after one unsuccessful attempt. Also, the

incidence of semi-permanent faults that can be cleared by

repeated reclosures is less likely than on HV systems. Multi-

shot schemes have, however, occasionally been used on EHV

systems, specifically to deal with bush fire faults prevalent in

Africa.

14.7 SINGLE-PHASE AUTO-RECLOSING

Single phase to earth faults account for the majority of

overhead line faults. When three-phase auto-reclosing is

applied to single circuit interconnectors between two power

systems, the tripping of all three phases may cause the two

systems to drift apart in phase, as described in Section 14.5.

No interchange of synchronising power can take place during

the dead time. If only the faulty phase is tripped,

synchronising power can still be interchanged through the

healthy phases. Any difference in phase between the two

systems will be correspondingly less, leading to a reduction in

the disturbance on the system when the circuit breaker

recloses.

For single-phase auto-reclosing each circuit breaker pole must

be provided with its own closing and tripping mechanism; this

is normal with EHV air blast and SF

breakers. The associated

tripping and reclosing circuitry is therefore more complicated,

as it must be inherently phase-selective.

On the occurrence of a phase-earth fault, single-phase auto-

reclose schemes trip and reclose only the corresponding pole of

the circuit breaker. The auto-reclose function in a relay

therefore has three separate elements, one for each phase.

Operation of any element energises the corresponding dead

timer, which in turn initiates a closing pulse for the appropriate

pole of the circuit breaker. A successful reclosure results in the

auto-reclose logic resetting at the end of the reclaim time,

ready to respond to a further fault incident. If the fault is

persistent and reclosure is unsuccessful, it is usual to trip and

lock out all three poles of the circuit breaker.

The above describes only one of many variants. Other

possibilities are:

x three-phase trip and lockout for phase-phase or 3-

phase faults, or if either of

the remaining pha

ses should

develop a fault during the dead time

x use of a selector switch to give a choice of single or

three-

phase reclosing

x combined single and three-phase auto-reclosing; single

phase to

earth faults initiate single-phase tripping and

reclosure, and phase-phase faults initiate three-phase

tripping and reclosure

Modern numerical relays often incorporate the logic for all of

the above schemes for the user to select as required. Use can

be made of any user-definable logic feature in a numerical

relay to implement other schemes that may be required.

The advantages of single-phase auto-reclosing are:

x the maintenance of system integrity

x on multiple earth systems, negligible interference with

the transmission of load. This is

because the current in

the unfaulted phases can continue to flow until the fault

is cleared and the faulty phase restored

The main disadvantage is the longer de-ionisation time

resulting from capacitive coupling between the faulty and

healthy lines. This leads to a longer dead time being required.

Maloperation of earth fault relays on double circuit lines owing

to the flow of zero sequence currents may also occur. These

are induced by mutual induction between faulty and healthy

lines (see Chapter 13 for details).

14.8 HIGH-SPEED AUTO-RECLOSING ON

LINES EMPLOYING DISTANCE SCHEMES

The importance of rapid tripping of the circuit breakers at each

end of a faulted line where high-speed auto-reclosing is

employed has already been covered in Section 14.6. Simple

distance protection presents some difficulties in this respect.

Owing to the errors involved in determining the ohmic setting

of the distance relays, it is not possible to set Zone 1 of a

distance relay to cover 100% of the protected line – see

Chapter 11 for more details. Zone 1 is set to cover 80-85% of

the line length, with the remainder of the line covered by time-

delayed Zone 2 protection.

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